Myelodysplastic syndrome (MDS) is characterized by ineffective hematopoiesis, most commonly of the erythroid lineage, resulting in a phenotype termed refractory anemia. In the 5q- syndrome, a subtype of MDS, a single genetic lesion, a heterozygous, interstitial deletion of Chromosome 5q, causes a highly reproducible clinical phenotype, though the molecular basis of this phenotype was previously unknown. In the previous funding period, we identified one protein-coding gene, RPS14, and one miRNA, miR-145, that contribute to abnormal hematopoiesis in the 5q- syndrome. The finding that RPS14 haploinsufficiency causes a block in erythropoiesis, the dominant phenotype of the 5q- syndrome, established a previously unrecognized link between the 5q- syndrome and Diamond Blackfan Anemia, a congenital disorder with a similar phenotype that is also caused by genetic inactivation of one allele of genes encoding ribosomal proteins. We found that haploinsufficiency for miR-145 causes increased expression of a key target gene, FLI-1, leading to increased megakaryocyte production and the characteristic hypolobated micromegakaryocytes found in this syndrome. In this renewal application, we aim to understand the molecular basis for the effects of RPS14 haploinsufficiency and combined haploinsufficiency for RPS14 and miR-145, and to examine the effects of these lesions on hematopoietic stem cells. In addition, having established an approach to the identification of key MDS genes within chromosomal deletions, we will apply our methodology to identify a key gene within the 7q deletion, another common genetic lesion in MDS. In Aim 1, we will investigate the mechanism whereby ribosomal haploinsufficiency leads to impaired erythropoiesis. Current evidence supports two non-exclusive hypotheses. The first possibility is that selective activation of p53 in the erythroid lineage causes cell cycle arrest and apoptosis, resulting in macrocytic anemia. Alternatively, or additionally, abnormal ribosome biogenesis could lead to dysregulated mRNA translation and altered production of specific proteins. We will examine both hypotheses in primary human bone marrow progenitor cells. In Aim 2, we will examine hematopoiesis in genetically engineered murine models with conditional haploinsufficiency of RPS14, miR-145, or the combination of RPS14 and miR-145. In particular, we will use these models to examine the effect of each lesion on hematopoietic stem cell function. In Aim 3, we will extend our RNA interference screening approach to identify additional genes that are critical for the pathogenesis of MDS. Having demonstrated the ability to use this approach to identify haploinsufficiency disease genes on Chromosome 5q, we will next focus this technology towards the identification of novel MDS genes on Chromosome 7q. In aggregate, these experiments will provide critical insight into the molecular basis of myelodysplastic syndrome.